153.9 Synthesis
Substantial progress has been made since AR4 in documenting and understanding change in the ocean. The major findings of this chapter are largely consistent with those of AR4, but in many cases statements can now be made with greater confidence because more data are available, biases in historical data have been identified and reduced, and new analytical approaches have been applied. Changes have been observed in a number of ocean properties of relevance to climate. It is virtually certain that the upper ocean (0 to 700 m) has warmed from 1971 to 2010 (Section 3.2.2, Figures 3.1 and 3.2). Warming between 700 and 2000 m likely contributed about 30% of the total increase in global ocean heat content between 1957 and 2009 (Section 3.2.4, Figure 3.2). Global mean sea level has risen by 0.19 to 0.21 m over the period 1901–2010. It is very likely that the mean rate was 1.7 to 1.9 mm yr –1 between 1901 and 2010 and increased to 3.2 to 3.6 mm yr –1 between 1993 and 2010 (Section 3.7, Figure 3.13). The rise in mean sea level can explain most of the observed increase in extreme sea levels (Figure 3.15). Regional trends in sea surface salinity have very likely enhanced the mean geographical contrasts in sea surface salinity since the 1950s: saline surface waters in evaporation-dominated regions have become more saline, while fresh surface waters in rainfall-dominated regions have become fresher. It is very likely that trends in salinity have also occurred in the ocean interior. These salinity changes provide indirect evidence that the pattern of evaporation minus precipitation over the oceans has been enhanced since the 1950s (Section 3.4, Figures 3.4 and 3.5]. Observed changes in water mass properties likely reflect the combined effect of long-term trends in surface forcing (e.g., warming and changes in evaporation minus precipitation) and variability associated with climate modes (Section 3.5, Figure 3.9). It is virtually certain that the ocean is storing anthropogenic CO2 and very likely that the ocean inventory of anthropogenic CO2 increased from 1994 to 2010 (Section 3.8, Figures 3.16 and 3.17). The uptake of anthropogenic CO2 has very likely caused acidification of the ocean (Section 3.8.2, Box 3.2). For some ocean properties, the short and incomplete observational record is not sufficient to detect trends. For example, there is no observational evidence for or against a change in the strength of the AMOC (Section 3.6, Figure 3.11). However, recent observations have strengthened evidence for variability in major ocean circulation systems and water mass properties on time scales from years to decades. Much of the variability observed in ocean currents and in water masses can be linked to changes in surface forcing, including wind changes associated with the major modes of climate variability such as the NAO, SAM, ENSO, PDO and the AMO (Section 3.6, Box 2.5). The consistency between the patterns of change in a number of independent ocean parameters enhances confidence in the assessment that the physical and biogeochemical state of the oceans has changed. This consistency is illustrated here with two simple figures (Figures 3.21 and 3.22). Four global measures of ocean change have increased since the 1950s: the inventory of anthropogenic CO2, global mean sea level, upper ocean heat content, and the salinity contrast between regions of high and low sea surface salinity (Figure 3.21). High agreement among multiple lines of evidence based on independent data and different methods provides high confidence in the observed increase in these global metrics of ocean change. The distributions of trends in subsurface water properties, summarized in a schematic zonally averaged view in Figure 3.22, are consistent both with each other and with well-understood dynamics of ocean circulation and water mass formation. The largest changes in temperature, salinity, anthropogenic CO2, and other properties are observed along known ventilation pathways (indicated by arrows in Figure 3.22), where surface waters are transferred to the ocean interior, or in regions where changes in ocean circulation (e.g., contraction or expansion of gyres, or a southward shift of the Antarctic Circumpolar Current) result in large anomalies. Zonally averaged warming trends are widespread throughout the upper 2000 m, with largest warming near the sea surface. Water masses formed in the precipitation-dominated mid to high latitudes have freshened, while water masses formed in the evaporation-dominated subtropics have become saltier. Anthropogenic CO 2 has accumulated in surface waters and been transferred into the interior, primarily by water masses formed in the North Atlantic and Southern Oceans. In summary, changes have been observed in ocean properties of relevance to climate during the past 40 years, including temperature, salinity, sea level, carbon, pH, and oxygen. The observed patterns of change are consistent with changes in the surface ocean (warming, changes in salinity and an increase in Cant) in response to climate change and variability and with known physical and biogeochemical processes in the ocean, providing high confidence in this assessment. Chapter 10 discusses the extent to which these observed changes can be attributed to human or natural forcing. Improvements in the quality and quantity of ocean observations has allowed for a more definitive assessment of ocean change than was possible in AR4. However, substantial uncertainties remain. In many cases, the observational record is still too short or incomplete to detect trends in the presence of energetic variability on time scales of years to decades. Recent improvements in the ocean observing system, most notably the Argo profiling float array, mean that temperature and salinity are now being sampled routinely in most of the ocean above 2000 m depth for the first time. However, sparse sampling of the deep ocean and of many biogeochemical variables continues to limit the ability to detect and understand changes in the global ocean.